High power lasers have important applications in the laser processing of industrial materials. Pulsed lasers, with peak powers exceeding 10 kW, are used in marking, engraving, cuffing, welding, and drilling applications. Continuous wave lasers with powers exceeding 500 W are used in cutting and welding applications. These high power lasers typically comprise more than one specialist optical fibre that have dissimilar optical properties. It is often difficult to make low optical loss splices between such fibres reliably.
High power industrial fibre lasers use amplifying optical fibres as the gain medium. For many applications, good beam quality is desired, and thus the amplifying optical fibres need to output single-mode or low-moded laser radiation. In order to achieve high power (>1 kW) and high peak powers (>10 kW), the amplifying fibre needs to be a large mode area fibre to reduce non-linear effects. This is especially the case in picosecond (ps) lasers which can have peak powers exceeding 100 kW. Such large mode area fibres are multimoded and are configured in the laser to output single mode radiation. Such fibres can have larger mode field diameters than fibres that connect with them, and thus tapering between the different fibre types is often required in order to achieve low loss splices. Additionally, certain amplifying fibres can experience photodarkening whereby the attenuation of the amplifying fibre increases over time. Elimination of photodarkening places constraints on dopant selection and dopant concentration within the amplifying fibre.
High power industrial lasers advantageously have optical fibre beam delivery systems for delivering the laser radiation from the laser to a work piece. Unfortunately, non-linear effects, such as Raman scattering and stimulating Raman scattering can limit the maximum output power and the length of the optical fibre beam delivery system, as well as impacting the ability of the laser to withstand back reflection from the work piece. The maximum length for some systems can be as small as 1 m to 2 m. This places serious limitations on the design of laser processing machines such as flat bed cutters, or the design of manufacturing shop floors, as well as on the architecture of the laser source itself. Non-linear effects are reduced with increasing core diameter, and thus optical fibres used in optical fibre beam delivery systems typically have larger core diameters than other signal fibres within the laser.
Prior art splices between dissimilar fibres either use dopant diffusion within the splice, by inserting an intermediate fibre between the two fibres being spliced, or by inserting or creating a physical taper of the outside of the fibre. Of these, the dopant diffusion is generally preferred as it can be carried out in the fusion splicer used for splicing the two fibres together. Tapering the outside of the fibre is used if the mode mismatch between the fibres is too great, or if the dopants do not diffuse reliably enough.
The use of intermediate fibres, such as graded index fibres, between the fibres in the splice is known. However this requires multiple splicing and cleaving operations, and accurate control of the length of the intermediate fibre, both of which add cost and complexity.
The degree of freedom of the dopant diffusion process is limited by the diffusion properties of the dopants. When splicing two fibres with a large difference in mode field diameter, for example greater than 50%, additional tapering of the outside of at least one of the fibres is required to obtain a low loss splice. However, even additional tapering cannot reduce losses to acceptable amounts if there is a large mismatch in numerical apertures between the fibres. Prior art dopant diffusion processes are often referred to as thermally expanded cores because diffusion is caused by heating, and the core dopants diffuse into the cladding, thus expanding the size of the core, and thus increasing the mode field area of the guided modes. Thermally expanded cores are used in splices between two optical fibres both having Gaussian shaped modes.
European Patent No. 1202090 describes an optical splice between a dispersion compensating fibre and a transmission fibre. The dispersion compensating fibre is a so-called “W-fibre” having a narrow fluorine doped ring around the core. The transmission fibre is a low-loss depressed cladding telecommunications fibre having multiple fluorophosphorus doped layers around the core. Splicing of the two fibres causes problems owing to the different diffusion rates of germania in the core and fluorine in the claddings between the two fibres. The solution is to use a bridge or intermediate fibre between the two fibres in order to form a low loss splice.
U.S. Pat. No. 6,336,749 describes a splice between a first fibre and a second fibre in which diffusion of the dopants in the second fibre causes its refractive index profile to converge optically to that of the first fibre. As a consequence of the diffusion region and its gradual longitudinal variation, the optical losses associated with the splice are relatively low, i.e. less than 0.2 dB at the operating wavelength, even when there is relatively high mismatch between the mode field diameters and mode field shapes (at a signal wavelength) in the respective fibres.
U.S. Pat. No. 5,381,503 discloses an optical waveguide comprising a core portion made of a light propagating material and a cladding portion, a first dopant and a second dopant being induced into said core portion, the first dopant having a function of increasing a refractive index of the light propagating material and having a first thermal diffusion coefficient to said light propagating material, the second dopant having a function of decreasing the refractive index of said light propagating material and having a second thermal diffusion coefficient to the light propagating material larger than the first thermal diffusion coefficient under a predetermined temperature. The patent teaches how to splice two single mode fibres together that have different mode field diameters. The patent does not teach how to design a mode field adaptor that can change the shape of the mode.
U.S. Pat. No. 6,742,939 discloses a fusion splice including a first optical fibre having a first mode field diameter and a first mode field diameter expansion rate. The splice further includes a second fibre having a second mode field diameter and a second mode field diameter expansion rate, wherein the second mode field diameter is lower than the first mode field diameter. The second fibre comprises a core, a cladding radially surrounding the core, and a zone of high concentration of fluorine between the core and the cladding. The rate of mode field diameter expansion of the first fibre is less than the rate of mode field expansion of the second fibre during the fusion splicing operation. The invention has application for splicing telecommunication single mode fibre to erbium doped fibre amplifiers.
A further limitation of dopant diffusion and tapering processes is that fibres with dissimilar mode shapes are difficult to splice together. The prior art solutions enable single mode fibres to be spliced to single mode fibres that have different mode field diameters with low loss. However, they are not suited for splicing fibres that have substantially different mode field diameters, or fibres whose refractive index profiles have substantially different shapes. For example, splicing from a Gaussian mode from a step index fibre to a doughnut mode from a ring core fibre would be difficult if not impossible with prior art techniques.
Further problems occur when splicing fibres having large mismatches in core diameter include dopants not diffusing rapidly enough. This results in sagging of the splice within the fusion splicer, and increased sensitivity to variations in the temperature profile across the fibres. Fibre sagging and increased temperature sensitivity result in unpredictable splicing processes that are unsuited to repeatable and reliable manufacturing. Such problems may be overcome in some cases with more expensive fusion splicing and ancillary equipment, but this is undesirable.
There is a need for an optical fibre that reduces or avoids the aforementioned problems.